This application claims priority to Japanese Patent Application No. JP 2001-245866, filed on Aug. 14, 2001, the disclosure of such application being herein incorporated by reference to the extent permitted by law.
1. Field of the Invention
The present invention relates to an active filter circuit to be utilized in a variety of electronic devices, for example for deriving a signal under a designated frequency.
2. Description of Related Art
In a variety of electronic devices, various active filters are used in order to isolate and extract a specific signal or to remove noises from signals of a wide range of frequencies. As an example of such active filters, a MOSFET-C filter (a low pass filter) 100 as shown in FIG. 7 is known as a configuration of a monolithic active filter (an active filter formed as a semiconductor integrated circuit).
A MOSFET-C filter operates in a non-saturated area of a MOSFET (Metal Oxide Semiconductor Field-effect Transistor: a field-effect transistor having a metal oxide film semiconductor structure). In other words, the MOSFET-C filter utilizes a linear operating area as a variable resistance, and it is known as a filter (a triode filter) which utilizes triode action as a variable resistance.
In other words, a MOSFET-C filter 100 as illustrated in FIG. 7 is configured such that circuit sections each having a plurality of MOSFET used as variable resistors, capacitance elements and an operational amplifier are connected to each other in multi-stage and a plurality of capacitance elements are used.
As described in an existing document (M. Banu and Y. Tsividis, xe2x80x9cAn Elliptic Continuous-Time CMOS Filter with On-Chip Automatic Tuning,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 20, no.6, pp1114-1121, December 1985), a MOSFET-C filter IC (integrated circuit) operable with a xc2x15 V power supply has been realized by Banu et al.
In recent years, however, voltage of a power supply for an IC has been reduced because of requirements for a more efficient use of a battery taking in consideration installation of the battery in a so-called a mobile terminal such as a mobile communications terminal. Also, because of miniaturization of the semiconductor fabricating process, there is a strong need for an IC to be operable with a single power supply of 2.7 V or less.
Generally, a MOSFET-C filter needs a power supply of relatively high voltage as compared, for example, with a Gm-C filter (temporally continuous filter) which is another method for designing a monolithic filter. This is because, in order to let a MOSFET used as a variable resistor perform a good linear action (a triode action), sufficiently high gate voltage (VG) must be applied to the gate terminal of the MOSFET.
The MOSFET-C filter, therefore, has a serious drawback with respect to lowering the voltage of the power supply in terms of circuit design. Even in a conventional power supply of 2.7 V, it is difficult to obtain a MOSFET-C filter of a higher dynamic range.
In order to solve the problem, as shown in FIG. 8, for example, a charge pump section (a charge pump circuit) 204 is provided within a frequency adjusting section 200 which supplies a gate voltage (VG) to a gate terminal of each MOSFET of the MOSFET-C filter. The MOSFET-C filter 100 may be driven and controlled by the gate voltage (VG) which is increased to a higher voltage than that of the power supply in the charge pump section 204.
As shown in FIG. 7, for example, since a terminal used for frequency adjusting in the MOSFET-C filter 100 is the gate terminal of the MOSFET, a simple charge pump section may easily provide a gate voltage higher than the power supply voltage, thus the MOSFET filter 100 may be applied to a filter circuit having a high dynamic range which is required in a receiving circuit of a mobile communications terminal.
In other words, as shown in FIG. 9, the MOSFET-C filter 100 and the frequency adjusting section 200 supplying the gate voltage (VG) thereto make it possible to configure an active filter circuit having a high dynamic range.
It is to be noted that the frequency adjusting section 200 as shown in FIG. 8 is a design of so-called DLL (Delay Locked Loop) which is configured to be locked when a predetermined phase difference, 90 degrees, for example, is caused between a phase of an output signal from a filter (a MOSFET-C filter for frequency adjusting) 201 having a function of a delay circuit and that of an input signal (reference clock signal CLK) to the frequency adjusting section 200.
In this case, the output signal from the filter 201 and an input signal to the frequency adjusting section 200 are multiplied by a multiplier 202. The multiplier 202 outputs a signal comprised of a doubled signal component and a DC component which are supplied to a loop filter 203. The loop filter 203 extracts only the DC component, which is then supplied to a charge pump section 204 as a control signal.
A voltage charging action (voltage increasing action) at the charge pump section 204 is carried out until the phase difference between the output signal from the filter 201 and the input signal to the frequency adjusting section 200 is a predetermined value (90 degrees, for example) so as to obtain a gate voltage of a target level.
When the phase difference between the output signal from the filter 201 and the input signal to the frequency adjusting section 200 reaches the predetermined value, the charge pump 204 is locked to supply the gate voltage (VG) of the target level to each gate terminal of the filter 201 of the frequency adjusting section 200 and the MOSFET-C filter 100, whereby the MOSFET-C filter is driven and controlled.
It is to be noted that, the following documents (1) and (2) describe how to increase a gate voltage of an active filter by means of a charge pump section:
(1). G. L. E. Monna, J. C. Sandee, C. J. M. Verhoeven, G. Groenewold, and A. H. M. van Roermund, xe2x80x9cCharge Pump for Optimal Dynamic Range Filters,xe2x80x9d Proceedings,1994 IEEE International Symposium on Circuits and System, vol.5,pp747-750,1994;
(2). A. Yoshizawa and Y. Tsividis, xe2x80x9cAn Anti-Blocker Structure MOSFET-C Filter For a Direct Conversion Receiver,xe2x80x9d Proceeding,2001 IEEE Custom Integrated Circuit Conference.
However, such a configuration in which the frequency adjusting section 200 having the charge pump section 204 drives a cut-off frequency control terminal (a gate terminal) of the MOSFET-C filter 100 may cause a clock signal for driving the charge pump section 204 to leak into the MOSFET-C filter 100 and present a problem in that a dynamic range of the MOSFET-C filter 100 is limited.
For example, noise from the frequency adjusting section 200 may be mixed into a gate terminal of the MOSFET-C filter 100 shown in FIG. 7, increasing the output signal from the MOSFET-C filter 100 to an excessively high level. If the signal coincides with timing of a clock signal and happens to be picked up, the dynamic range of the MOSFET-C filter, resulting in a worse characteristic of the MOSFET-C filter.
Further, in a receiving circuit of a mobile communications terminal, for example, less power consumption in the circuit is preferable in an effort to extend stand-by time for receiving calls. However, in order to control the cut-off frequency of an active filter, it is necessary for an analogue control to keep a dedicated circuit such as the frequency adjusting section 200 as shown in FIG. 8, for example, constantly active, thus resulting in a burden for reduction of power consumption.
Still further, when a digital control is used in order to control a cut-off frequency of an active filter, a D/A (digital to analogue) converter is required. As a result, the size of the circuit is increased and also its power consumption is increased.
In view of the above description of the existing problems related to the conventional art, the present invention provides an active filter circuit capable of reducing its power consumption without presenting a problem such as limit of its dynamic range.
In order to provide such an active filter circuit, an active filter circuit according to first aspect of the present invention includes: an active filter section wherein a cut-off frequency thereof is made variable upon utilizing at least one field effect transistor element having metal oxide film semiconductor structure as a variable resistor; a charge pump section for supplying voltage for controlling a variable resistance against all gate terminals of the field effect transistor element; a switch section for short-circuiting or opening contact between the gate terminals and an output terminal of the charge pump section; a capacitance element connected between ground and the gate terminals; a frequency adjusting section for supplying a control signal to the charge pump section in order to generate voltage for adjusting frequency of the active filter section; and a cut-off frequency judging section for controlling ON/OFF of the switch section according to an adjusting status of a cut-off frequency of the active filter section.
According to the active filter circuit of the first aspect of the present invention, the switch section and the capacitance element are provided between the active filter section and the charge pump section. Short-circuiting/opening (ON/OFF) the switch section is controlled by the cut-off frequency judging section. The cut-off frequency judging section controls ON/OFF of the switch section according to an adjusting status of the cut-off frequency of the active filter.
In particular, the cut-off frequency judging section judges whether or not an increased voltage at the charge pump section reaches a predetermined level, for example according to a control signal from the frequency adjusting section to the charge pump section. When the voltage increase is achieved, the switch section is set to OFF and when the increment is not achieved, the switch section is kept ON.
The switch section and the capacitance element between ground and the gate terminals of the active filter section establish a so-called sample and hold circuit, whereby the voltage applicable to the gate terminals is maintained while the switch section is OFF so as to operate the active filter section properly.
As described above, even when the switch section is OFF, that is, even in the case of opening contact between the active filter section and the charge pump section, the active filter section can be operated properly. Further, by setting the switch section to OFF, noises such as a clock signal are stopped from leaking to the active filter section, and the problem of limited dynamic range of the active filter section may be avoided.
An active filter circuit according to a second aspect of the present invention includes an active filter circuit as recited in the first preferred embodiment in which the charge pump section is stopped from operating when the switch section is under an OFF status.
According to the active filter circuit of the second aspect of the invention, when the switch section is under the OFF status, the charge pump section does not have to be operated and therefore the charge pump section is at least stopped from operating. The stop of the charge pump section may be performed by stopping power supply to the charge pump section, whereby power consumption of the active filter circuit may be reduced.
An active filter circuit according to a third aspect of the present invention includes the active filter circuit as recited in the first preferred embodiment, in which the frequency adjusting section is stopped from operating when the switch section is under the OFF status.
According to the active filter circuit of the third aspect of the present invention, when the switch section is under the OFF status, the frequency adjusting section is at least stopped operating as a boosting operation has been completed at the charge pump section. The stop at the frequency adjusting section may be performed by stopping power supply to the frequency adjusting section, whereby the power consumption of the active filter circuit may be reduced.
An active filter circuit of fourth aspect of the present invention includes the active filter circuit as recited in the first preferred embodiment, in which operations of the charge pump section and the frequency adjusting section are stopped from operating when the switch section is under the OFF status.
According to the active filter circuit of the fourth aspect of the present invention, when the switch is under the OFF status, neither the charge pump section nor the frequency adjusting section has to be operated and therefore the charge pump section and the frequency adjusting section are stopped from operating.
The stop of these circuits may be performed by stopping power supply to each circuit, whereby the power consumption of the active filter circuit may be reduced. Further, the charge pump section itself and the frequency adjusting section itself are stopped from operating, whereby noise leakage such as a leaked clock signal from these circuits to the active filter can be stopped.
An active filter circuit according to a fifth aspect of the present invention includes: an active filter section wherein a cut-off frequency thereof is made variable upon utilizing at least one field effect transistor element having metal oxide film semiconductor structure as a variable resistor; a charge pump section for supplying voltage for controlling a variable resistance against all gate terminals of the field effect transistor element; a charge pump section capable of switching from a control voltage supplying operation for supplying voltage for controlling a variable resistance against all gate terminals of the field effect transistor element to a high output impedance status preventing supply of voltage, wherein the charge pump section is capable of maintaining the voltage to be supplied to the active filter section when under the high output impedance status; a frequency adjusting section for supplying a control signal to the charge pump section in order to generate voltage for adjusting frequency of the active filter section; and a cut-off frequency judging section for controlling the control voltage supplying operation of the charge pump section and the high output impedance status based on an adjusting status of a cut-off frequency of the active filter section.
According to the active filter circuit as recited in the fifth aspect of the present invention, the charge pump section for supplying a voltage to the gate terminals of the active filter section is configured to be able to switch from a control voltage supplying operation for supplying a voltage to the gate terminals of the active filter to a high output impedance status which does not provide a voltage, and to maintain a voltage to the active filter section when under the high output impedance status.
The switching from the control voltage supplying operation to the high output impedance status is controlled by the cut-off frequency judging section. In particular, the cut-off frequency judging section judges whether or not an increased voltage at the charge pump section reaches a predetermined level according to a control signal from the frequency adjusting section to the charge pump section, for example. When the increase in voltage is achieved, the high output impedance status is selected.
As a result when in the high output impedance status, a voltage is not supplied from the charge pump section, thereby preventing a noise such as a clock signal leaked out of the charge pump section from leaking to the active filter section and also preventing the active filter section from limiting its dynamic range.
An active filter circuit as a sixth aspect of the present invention includes the active filter circuit as recited in the fifth aspect with the charge pump section stopped from operating when the charge pump section is under the high output impedance status.
According to the active filter circuit as recited in the sixth aspect of the present invention, when the charge pump section is under the high output impedance status, the charge pump section does not have to supply a voltage to the active filter section, and the voltage to be supplied to the active filter section need only be maintained and therefore the charge pump section is stopped from operating, thereby reducing the power consumption of the active filter circuit.
An active filter circuit according to a seventh aspect of the present invention includes the active filter circuit as recited in the fifth aspect, with the charge pump section and the frequency adjusting section stopped from operating when the charge pump section is under the high output impedance status.
According to the active filter circuit as recited in the seventh aspect of the present invention, when the charge pump section is under the high output impedance status, the charge pump section does not have to supply a voltage to the active filter and the voltage to be supplied to the active filter need only be maintained.
As a result, the frequency adjusting section does not have to be operated, and therefore the frequency adjusting section is stopped from operating, thereby reducing the power consumption of the active filter circuit.
An active filter circuit according to an eighth preferred embodiment of the present invention includes the active filter circuit as recited in the fifth aspect with the charge pump section and the frequency adjusting section stopped from operating when the charge pump section is under the high impedance status.
According to the active filter circuit as recited in the eighth aspect of the present invention, when the charge pump section is under the high output impedance status, neither the charge pump section nor the frequency adjusting section has to be operated. Therefore, the charge pump section and the frequency adjusting section are stopped from operating when the charge pump section is under the high output impedance status.
As a result, the power consumption of the active filter circuit may be reduced. Further, when both the charge pump section and the frequency adjusting section are stopped from operating, a noise from these circuits does not leak into the active filter section, and the dynamic range of the active filter section is not limited by such a noise.
An active filter circuit according to a ninth preferred embodiment of the present invention includes the active filter circuit as recited in the fifth aspect, with the charge pump section including a charge section for increasing a charge pump output voltage and a discharge section for decreasing the charge pump output voltage; the charge section including voltage multiplying circuit for generating a voltage by utilizing a charge holding characteristic of a rectifying element and a capacitance element; and the discharge section including a current circuit for extracting an amount of charge from a load capacitance of the charge pump section.
According to the active filter circuit as recited in the ninth aspect of the invention, the charge pump section includes a voltage multiplying circuit as a charge section and a current circuit as a discharge section. The current circuit makes the charge pump section operate at a high impedance by taking an amount of charge out of a load capacitance of the charge pump section, which is more simple than short-circuiting a transmission line of the gate voltage which is boosted above its power supply or opening the switch section such that the charge pump section is under the high output impedance status.
An active filter circuit as recited according to a tenth aspect of the present invention includes the active filter circuit as recited the ninth aspect, with the high output impedance status of the charge pump section established by stopping the voltage multiplying circuit and current cut-off by the current circuit.
According to the active filter circuit as recited in the tenth aspect of the present invention, by stopping the voltage multiplying circuit from operating and a current cut-off by the current circuit, the charge pump section ensures the high output impedance, and the power consumption of the active filter circuit is reduced by stopping the voltage multiplying circuit from operating.
As described above, according to the active filter circuits according to the present invention, power consumption may be reduced by decreasing a rate of operation of the frequency adjusting section for adjusting a frequency of the active filter section, thereby providing an active filter circuit suitable for mounting in a mobile terminal device such as a mobile phone terminal where efficient utilization of batteries is required.
Further, by opening the frequency adjusting section and the active filter section, and by stopping a circuit section of the frequency adjusting section, the target active filter section is prevented from degradation of dynamic range, which is caused by a clock signal leaked from the frequency adjusting section.